Martin Bjorklund <[email protected]> writes:
> Qin Wu <[email protected]> wrote:
>> -----邮件原件-----
>> 发件人: netmod [mailto:[email protected]] 代表 Ladislav Lhotka
>> 发送时间: 2018年10月22日 21:12
>> 收件人: Martin Bjorklund
>> 抄送: [email protected]
>> 主题: Re: [netmod] xpath expressions in JSON
>>
>> On Mon, 2018-10-22 at 14:56 +0200, Martin Bjorklund wrote:
>> > Ladislav Lhotka <[email protected]> wrote:
>> > > Martin Bjorklund <[email protected]> writes:
>> > >
>> > > > Hi,
>> > > >
>> > > > Going back to the most urgent issue, what is this WG's
>> > > > recommendation for the subscribed-notifications draft in NETCONF
>> > > > wrt/ their usage of
>> > > > yang:xpath1.0 in filters?
>> > > >
>> > > > To summarize:
>> > > >
>> > > > We already have
>> > > >
>> > > > o instance-identifier in XML uses prefixes from the XML document
>> > > > o instance-identifier in JSON uses module names as prefixes
>> > > > o XPath in NETCONF filter uses prefixes from the XML document
>> > > > o XPath in JSON query filter uses module names as prefixes
>> > > >
>> > > >
>> > > > Alternative A:
>> > > > --------------
>> > > >
>> > > > Use different encodings for "stream-xpath-filter" as well,
>> > > > depending on if it is XML or JSON.
>> > > >
>> > > > We would do in SN:
>> > > >
>> > > > o If the node is encoded in XML, the set of namespace
>> > > > declarations are those in scope on the
>> > > > 'stream-xpath-filter' leaf element.
>> > > >
>> > > > o If the node is encoded in JSON, the set of namespace
>> > > > declarations is the set of prefix and namespace pairs
>> > > > for all supported YANG modules, where the prefix is
>> > >
>> > > Is "supported" the same as "implemented", or something else?
>> >
>> > It should be "implemented".
>> >
>> > > > the YANG module name and the namespace is as defined
>> > > > by the "namespace" statement in the YANG module.
>> > > >
>> > > > Pro: the format is consistent within each encoding.
>> > > >
>> > > > Con: unclear how to handle other encodings.
>> > > > Con: we keep using context-depending encodings.
>> > >
>> > > Con: XPath expressions in JSON can get pretty long (I assume it's not
>> > > just an instance identifier but may contain predicates etc.). We
>> > > cannot use the trick with the default namespace as in YANG, so all
>> > > data node names will have to carry the prefix.
>> >
>> > Yes.
>> >
>> > > > We could probably add that CBOR uses the same representation as JSON.
>> > > >
>> > > > Example in XML:
>> > > >
>> > > > <stream-xpath-filter
>> > > > xmlns:if="urn:ietf:params:xml:ns:yang:ietf-interfaces"
>> > > > xmlns:ip="urn:ietf:params:xml:ns:yang:ietf-ip">
>> > > > /if:interfaces/if:interface/ip:ipv4
>> > > > </stream-xpath-filter>
>> > > >
>> > > > Example in JSON:
>> > > >
>> > > > "stream-xpath-filter":
>> > > >
>> > > > "/ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4"
>> > > >
>> > > >
>> > > >
>> > > > Alternative B:
>> > > > --------------
>> > > >
>> > > > Use a non-context depending encoding, with the module name as prefix.
>> > > >
>> > > > We would do in SN:
>> > > >
>> > > > o The set of namespace
>> > > > declarations is the set of prefix and namespace pairs
>> > > > for all supported YANG modules, where the prefix is
>> > > > the YANG module name and the namespace is as defined
>> > > > by the "namespace" statement in the YANG module.
>> > > >
>> > > > Pro: the format is independent from the protocol encoding
>> > > >
>> > > > Con: in XML, this leaf is treated differently from other XPath
>> > > > expressions, such as get-config filter and nacm rules.
>> > > >
>> > > > Example in XML:
>> > > >
>> > > > <stream-xpath-filter>
>> > > > /ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4
>> > > > </stream-xpath-filter>
>> > > >
>> > > > Example in JSON:
>> > > >
>> > > > "stream-xpath-filter":
>> > > >
>> > > > "/ietf-interfaces:interfaces/ietf-interfaces:interface/ietf-ip:ipv4"
>> > > >
>> > > >
>> > > > My proposal is A. I think it is more important with consistency
>> > > > within each encoding than across encodings.
>> > >
>> > > I would suggest to consider declaring prefixes & namespaces
>> > > explicitly in the data, as in the schema mount document. It is
>> > > independent of encoding and the expressions can be kept short. In
>> > > fact, one of the namespaces can be declared as default, so this use
>> > > of XPath would then be very similar to YANG.
>> >
>> > Ok, so this is another alternative that works today, and achieves the
>> > goal of being encoding-independent. It is still context-dependent
>> > though.
>>
>> Yes, every module that uses XPath in data will have to deal with this. There
>> may potentially be multiple independent prefix declarations (this is
>> actually a con).
>>
>> >
>> > BTW, when used in filters, it is nice to let an unprefixed name to
>> > match any namespace; i.e., treat "foo" as syntactic sugar for
>> > "local-name(.) = 'foo'". ("*:foo" is not legal...)
>>
>> Hmm, I think this is a bad idea because it departs even further from the
>> original XPath semantics. Such chameleon names should IMO be pretty rare,
>> and if they are needed, local-name() is always available.
>>
>> [Qin]: Agree with Lada, Referencing RFC8407, section 4.6.2, I think the
>> below guideline is relevant.
>> "
>> The "local-name" function SHOULD NOT be used to reference local names
>> outside of the YANG module that defines the must or when expression
>> containing the "local-name" function. Example of a "local-name"
>> function that should not be used:
>>
>> /*[local-name()='foo']
>
> This guideline is for must/when expressions *within* YANG modules.
>
> I'm talking about a different use case, namely filtering. It is
> pretty convenient for users to send a filter:
>
> /interfaces/interface[name='eth0'/ipv4
This is impossible if we want to call it XPath. With an explicit
namespace/prefix declaration, for example
"namespace": [
{
"prefix": "if",
"uri": "urn:ietf:params:xml:ns:yang:ietf-interfaces",
"default": true
},
{
"prefix": "ip",
"uri": "urn:ietf:params:xml:ns:yang:ietf-ip"
}
]
it would be
/interfaces/interface[name='eth0']/ip:ipv4
which is not too bad either.
Lada
>
> and get back what they expect. Even in the rare case of local name
> clashes, this filter works and gives back what was expected (+
> additional nodes).
>
> I have no plans on writing up this as a proposal; I'm just pointing
> out that when XPath is used in filters, this is convenient.
>
>
> /martin
--
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67
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